We are interested in analyzing the interaction between mammalian cells and viruses. The application of genetics to this problem has been quite difficult, in part because of the lack of a stringent conditional lethal system. For this reason we are attempting to isolate nonsense suppressors in cultured mammalian cells. The nonsense suppressor system would be valuable in the analysis of mammalian viruses both because it should be non-leaky and because it will permit the direct biochemical identification of the mutant gene product. For this purpose we have isolated 500 independent cell lines lacking hypoxantine guanine phosphoribosyl transferase activity. These cell lines have been screened for immunologically cross reacting material (CRM). About 40 percent of the HGPRT cell lines were CRM , as measured by a specific radioimmune precipitation assay. We feel the CRM cell lines probably contain nonsense mutants. Revertants of these CRM minus lines will be assayed in vitro for suppressor tRNA. The in vitro system was derived from the parental L cell line and is programmed with RNA from a derivative of Q beta containing an amber mutation in the viral coat protein gene. The system responds to suppressor tRNA isolated from E. coli and was used to prove that yeast super-suppressors contain nonsense suppressor tRNA. The in vitro system will be used in further studies of the mechanism of suppression in yeast. The HGPRT mutants also afford an opportunity to study selective degradation of abnormal proteins in mammalian cells. These studies will be extended using both biochemical and genetic approaches.